Update nokogiri gem (#1108)

* Fixes security warning.
* Add `.ruby-version` hint for rbenv.
Signed-off-by: Ben Kochie <superq@gmail.com>

.ruby-version

+2.4.4

Gemfile.lock

@@ -39,22 +39,20 @@ GEM
       rb-inotify (>= 0.9)
     lumberjack (1.0.9)
     method_source (0.8.2)
-    mini_portile (0.6.0)
+    mini_portile2 (2.3.0)
     multi_json (1.11.2)
     nanoc (3.7.3)
       cri (~> 2.3)
     nanoc-cachebuster (0.3.1)
       nanoc (>= 3.3.0)
-    nokogiri (1.6.3.1)
-      mini_portile (= 0.6.0)
-    posix-spawn (0.3.9)
+    nokogiri (1.8.4)
+      mini_portile2 (~> 2.3.0)
     pry (0.10.1)
       coderay (~> 1.1.0)
       method_source (~> 0.8.1)
       slop (~> 3.4)
-    pygments.rb (0.6.0)
-      posix-spawn (~> 0.3.6)
-      yajl-ruby (~> 1.1.0)
+    pygments.rb (1.2.1)
+      multi_json (>= 1.0.0)
     rack (1.5.2)
     rb-fchange (0.0.6)
       ffi
@@ -79,7 +77,6 @@ GEM
     thor (0.19.1)
     timers (4.0.1)
       hitimes
-    yajl-ruby (1.3.0)
 
 PLATFORMS
   ruby
@@ -99,3 +96,6 @@ DEPENDENCIES
   rb-inotify
   redcarpet
   rspec
+
+BUNDLED WITH
+   1.16.2